1. Field of the Invention
The present invention relates to a photosensor for use in a wide variety of photoelectric converters for image information processing, such as facsimile transmitters and receivers and character readers.
2. Description of the Prior Art
The hitherto used photosensors of one-dimensional photodiode type comprising cyrstalline silicon have drawbacks in that the length of array is limited because of restrictions on the size of a producible single crystal of silicon and on the technique of fabrication thereof, and in that the yields thereof are low. Accordingly, when the original to be read is written on a relatively large width paper, e.g. A-4 size paper (210 mm wide), the reading is generally performed through formation of a reduced image of the original picture on the photosensor using a lens system.
When such an image-size reducing optical system is used, reduction of the size of the light receiving system will be difficult, and the area of each picture cell of the photosensor needs to be small in order to maintain the resolving power and therefore a large quantity of light is required to produce a sufficient signal current. Thus, such photosensors are presently used for readers of low-speed type, which take a long reading time, or in readers for which high resolving power is not required.
In view of the above, a photoconductor type of photosensor has been proposed recently in which amorphous silicon (hereinafter designated as a-Si) is utilized as a photoelectric transducing element. A large area or large length photosensor of this type can readily be prepared since this can be made by vacuum deposition of an a-Si thin layer on a glass substrate. In consequence, large width originals can also be read with this a-Si type of photosensor without using any image-size reducing optical system, and the size of such a reader can be reduced with ease.
However, the hitherto proposed photosensors of a-Si photoconductor type still admit of improvement in performance characteristics and production costs. For instance, when the usual glass plate is used as the substrate, an alkali metal ion contained in the glass will diffuse into the a-Si layer to react with the a-Si, deteriorating the photoelectric transducing performance of the a-Si layer. For the purpose of eliminating this drawback, the metal ion diffusion into the a-Si layer is prevented as far as possible, according to the prior art, by using a glass plate of low alkali metal ion content, for example, glass plate #7059 (barium borosilcate glass; alkali content ca. 0.2 wt %), Pyrex glass plate #7740 (borosilicate glass), or Vycor glass plate #7913 (silica glass content 96 wt %), supplied by Corning Glass Co. However, these glass plates of low alkali content are expensive and their surfaces need to be polished because of their inferior smoothness. This leads to high costs of photosensor production.